MPI procedures sometimes assign a special meaning to a special value
of an
argument. For example, tag is an integer-valued argument of
point-to-point communication operations, that can take a special wild-card
value, MPI_ANY_TAG.
MPI_ANY_TAG
Such arguments will have a range of regular values, which is a proper
subrange
of the range of values of the corresponding type of the variable.
Special values (such as MPI_ANY_TAG)
will be outside the regular range. The range of regular values can
be queried using environmental inquiry
functions (Chapter ).

MPI also provides predefined named constant handles, such as
MPI_COMM_WORLD, which is a handle to an object that represents all
MPI_COMM_WORLD
processes available at start-up time and allowed to communicate with
any of them.

All named constants, with the exception of MPI_BOTTOM in
MPI_BOTTOM
Fortran, can be used in initialization expressions or assignments.
These constants do not change values during execution. Opaque objects
accessed by constant handles are defined and do not change value
between MPI initialization (MPI_INIT() call) and MPI
completion (MPI_FINALIZE() call).